1. Field of the Invention
The present invention relates to a roaming method of a mobile communication device, and more particularly, to a method and apparatus for realizing a high-speed Inter-Radio Access Technology (RAT) Code Division Multiple Access (CDMA) handover by using Inter-RAT idle roaming when a mobile communication device is moved from another mobile communication system such as Wideband Code Division Multiple Access (WCDMA) system and Global Standard for Mobile communications (GSM) system to a CDMA system.
2. Description of the Related Art
In the current society, a communication device is used as a long-distance communication device by many people. Particularly, since the mobile communication device provides a function of conveniently communicating while a user moves regardless of a location of the user, use of the mobile communication device is generally regarded as a necessity.
Since current mobile communication devices have a highly functional processor and increased memory resources, many functions in diverse fields in addition to conventional mobile communication services are provided.
Accordingly, various mobile communication services are developed and used. Since a user uses a mobile communication device in various locations, the user often uses the mobile communication device while moving through areas supporting at least two different mobile communication services. Accordingly, a roaming operation according to a move from a current mobile communication system area to another mobile communication system area is performed. For example, there is a case in which a mobile communication device is moved from a WCDMA system to a CDMA system.
FIG. 1 is a flowchart illustrating a conventional method of switching into a CDMA service while using a WCDMA service.
Referring to FIG. 1, a conventional mobile communication device has a CDMA modem and a WCDMA modem to use a mobile communication modem corresponding to each mobile communication service area in order to use at least two mobile communication services. A process required when moving from a WCDMA service area to a CDMA service area is as follows.
The mobile communication device is located in the WCDMA service area and uses a WCDMA service (S101). In this case, since the CDMA modem is not used, the CDMA modem is maintained in an offline mode such as a power-off mode or a low power mode (LPM) and only the WCDMA modem is active (S102). The mobile communication device is moved to the CDMA service area (S103). As an example of a mobile communication device operation, a handover message from a UTRAN is received by a WCDMA procedure and a CDMA system is detected for switching to the CDMA service. The mobile communication device determines idle roaming to the CDMA service (S104). The method of determining the idle roaming may vary with implementation methods of system providing companies and device manufacturing companies. For example, a method of cutting-off the WCDMA system and accessing the CDMA system is used.
The mobile communication device enables the CDMA modem to be activated into an online mode (S105) and drives a power-up registration timer defined as a T57m timer (S106). In this case, the T57m is a constant from CDMA2000 volumes of CDMA2000 standards and may be 20 seconds. Also, the mobile communication device can not perform a registration for a period of time in which the T57m timer operates according to CDMA2000 communication standards.
The mobile communication device receives an overhead message from the CDMA system (S107). However, since the T57m timer is not canceled, registration in the CDMA system is not performed (S108). In the case the mobile communication device utilizes the CDMA service, the registration in the CDMA system has to be initially performed. When the T57m timer is canceled (S109), the mobile communication device checks a registration state and increases a power of a transmitter to perform the registration in the CDMA system (S110). If the registration is successful after increasing the power, the mobile communication device can then use the CDMA service and the WCDMA modem is established to be switched into an offline mode such as a power-off mode and a low power mode (S111), and uses the CDMA service (S112).
As shown in FIG. 1, in the conventional mobile communication device, the CDMA modem has to be switched into the online mode and perform a process such as power-up registration in order to perform the idle roaming for a handover from the WCDMA service to the CDMA service. Accordingly, a process of registering for the CDMA service may not be performed until the power-up registration timer is canceled. For example, when the mobile communication device is moved to the CDMA service area while using a WCDMA data service, the WCDMA data service can not be used while the T57m timer operates and continuity of service may not be secured if a quick handover to the CDMA service is not performed, due to an end of a application service that is time-out before the T57m.
Also, when roaming has to be performed for a handover from the WCDMA service to the CDMA service, a pilot signal of the WCDMA system is too low to maintain the WCDMA service. In this case, if the registration process with respect to the CDMA service is not quickly performed, the continuity of the mobile communication service may not be provided to a user because a success rate of receiving or transmitting is decreased.
Also, since the registration process for a roaming operation is not defined in the current CDMA standard, processes such as the power-up and registration must be performed, thereby delaying completion of a registration process due to actually performing the processes.